The invention relates to a rotor for an electric motor, in particular for a multi-phase or single-phase electric motor, comprising a permanent-magnetic ceramic rotor cylinder made of an imperforate solid material and provided with moulded-on plastic elements on at least the cylinder end faces, which elements carry axially projecting shaft portions for rotatably journalling the rotor.
Such a rotor is known from U.S. Pat. No. 4,587,450. The two moulded-on plastic elements of this rotor are interconnected externally of the rotor cylinder.
Ceramic rotor materials are subject to substantial firing shrinkage, which can give rise to cracking. Particularly in the case of high-grade ceramic magnet materials shrinkage and cracking effects lead to defects and rejects.
U.S. Pat. No. 4,587,450 describes rotor cross-sections varying between round and angulate. An angulate ceramic rotor poses production-engineering problems; it requires a series of expensive grinding operations. The well-known round cylindrical ceramic rotor can be manufactured more economically but in the case of higher motor-power ratings the moulded-on plastic elements cannot be connected firmly enough to withstand the torques to be transmitted. Torque transmission problems do not arise with the rotors described in DE 36 12 724 A1 and JP Patent Application 54-140 109, because these are designed only for low power ratings in clocks.
It is an object of the invention to provide a rotor of the type defined in the opening paragraph, which provides a torsion-proof connection between the rotor and moulded-on plastic elements, even for the transmission of larger torques.